Apparatus for making pellets usable as aggregate or filler

ABSTRACT

Waste gas from a coal-fired power plant is scrubbed with a lime-water mixture to form a sludge whose water content is reduced and which is mixed with a binder such as water glass. This mixture is then pelletized to produce hard water-stable material suitable for use as an aggregate or filler. The waste gas may be stripped of its fly ash and this ash added to the sludge to thicken it.

This is a division of application Ser. No. 593,670, filed July 7, 1975.

FIELD OF THE INVENTION

The present invention relates to an apparatus for making pellets usableas aggregate or filler. More particularly this invention concerns such asystem and plant for using the pollutants in the waste gas of acoal-fired power plant as the starting material for such pellets.

BACKGROUND OF THE INVENTION

In order to minimize air pollution by power plants it is known toseparate from the waste gas of such plants the particulate material orfly ash in a cyclone or the like and mix it with a binder. This binderis then agglomerated with or without the addition of other fillers andconverted into briquets. In such a system the fly ash itself constitutespart of the aggregate of such briquets.

This has proven to be successful but does not eliminate gaseouspollutants, such as SO₂, SO₃, and HF, which must be removed from thewaste gas. It is known in order to remove these aforementioned gaseouspollution substances to admix the flue gas with lime and then scrub itwith water. The flue gas is treated with an amount of lime, in form ofCaO or Ca(OH)₂, necessary to eliminate the gaseous pollutants. Afterthis procedure and after washing a sludge is obtained which mainlycomprises CaSO₄ and CaSO₃, and in case of a coal burning plant also fluedust or fly ash.

In big plants this sludge accumulates in large quantities. Its disposalcreates a problem. A purposeful utilization has not been suggested.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide animproved apparatus for treatment of the pollutants of flue gas from apower plant.

Another object is to provide an apparatus for using the gaseouspolluting substances from flue gases of power plants to produce pelletssuitable for use as aggregates or filling material.

SUMMARY OF THE INVENTION

These objects are obtained according to the present invention byproviding a method of producing pressure-resistant and substantiallywater-stable pellets suitable for use as aggregates or filler.

In accordance with the present invention therefore the flue gas of powerplants is scrubbed after being admixed with line. The water content ofthe sludge so obtained is reduced and the concentrated sludge is treatedwith a hydraulic binder or with water glass. The mixture is pelletized,and the pellets are finally hardened in air, and/or in an autoclave,and/or are burnt in a neutral atmosphere.

In accordance with the present invention the aforementioned sludge canbe pelletized by reducing its water content and mixing the sludge with asuitable binder, especially a hydraulic binder such as cement, otherhydraulic binders, e.g. gypsum, clay, lime or mixtures thereof, can alsobe used. Water glass is also usable as a binder. Surprisingly, evenafter being hardened only by air the pellets have considerable strengthand water stability which increases with time. Strength and waterstability can be further increased by burning. The amount of binderdepends on the desired final strength, and amounts to approximately1/100 to 1/5 of the water-content-reduced sludge.

The pellets produced can be used for various purposes, such as:

filling of shut-down gravel pits, shut-down mining tunnels, or otherundesired excavations,

filling undesired water areas such as excavated lakes, for thereclamation of land at the ocean, and for consolidation of boggysubsoils,

for road-building either comminuted or whole, or if necessary mixed withother aggregates,

in concrete, if necessary mixed with gravel if the desired concretestrength is not too high, and

for untreated sludge, also sludge obtained from flue gas of power plantsafter undergoing washed, either deposited homogeneously or in layers.

In all cases, especially where the pellets are used broken or unbrokenas aggregates, burnt pellets find a useful purpose. The burning servesto evaporate the water in the pellets and to sinter at least the surfaceof the pellets.

According to a feature of the invention the sludge is sedimented to awater content of between 20% and 40%, or approximately 30%, thereafterit is heat-dried to a water content of between 10% and 20%, orapproximately 15%, and then it is mixed with a hydraulic binder or withwater glass for pelletizing.

Where the flue gas of coal-burning power plants is treated, a modifiedmethod may be used. The flue gas is stripped of flue dust by means ofelectrostatic precipitation or a cyclone before it is scrubbed. The flyash so removed is added back to the sludge later, preferably togetherwith the hydraulic binder or water glass before pelletizing. If fly ashis added together with the hydraulic binder the water content of thesludge should be 30% before mixing, and water can be added in amountsneeded to assure proper pelletizing.

According to another feature of the present invention a plant isprovided to carry out the aforedescribed method. The basic arrangementof the plant comprises a scrubber with means for injecting pulverulentlime as a dispersing agent, a decanter for sedimenting the sludge soformed, a mixer with means for injecting a hydraulic binder or waterglass into the concentrated sludge, and a pelletizing device, e.g. apelletizing drum.

Means are provided to conduct the sludge from the scrubber to thedecanter, and to carry the decanted sludge to a heat dryer, from thedryer to the mixer, and from the mixer the sludge is carried to thepelletizer.

When a fly-ash remover is installed before the washing installation,especially in case of a coal-burning plant, the interposition of a heatdryer is unnecessary. The sludge coming from the washing installation ismixed with the fly ash stripped from the waste gas before scrubbingthereof, together with a hydraulic binder and a pelletizable mixture isobtained. In this case a drying installation is installed after thepelletizer.

A plant designed to carry out the method of the present invention worksvery economically if the drying installation or the burning installationis constructed as a traveling-grate device or as a rotary kiln and isdirectly or indirectly heatable with the flue gas or part of the fluegas from the power plant. The pelletizing drum is always provided with adevice for water supply.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages of the inventionwill become more readily apparent from the following description,reference being made to the drawing in which:

FIG. 1 is a diagrammatic view illustrating a plant for carrying out themethod according to this invention; and

FIG. 2 is a view similar to FIG. 1 illustrating another plant inaccordance with this invention.

SPECIFIC DESCRIPTION

The plants of FIGS. 1 and 2 both comprise a scrubber 1 as described onpage 14-37 of the Chemical Engineers' Handbook by J. Perry (McGraw-Hill1963) with a pump 2 for introducing a mixture of lime and water. Thescrubbed gas issues from the scrubber 1 at 14 and the sludge at 15.Waste gas from a coal-fired power plant 8 is fed to the scrubber 1.

A settling tank or decanter 3 is connected to the output 15 of thescrubber 1 and has an output 14 connected to a heater-dryer 9 whoseoutput 17 in turn is fed to a mixer 4. A pump 5 feeds a hydraulic binderto the mixer 4. The Chemical Engineers' Handbook (op.cit.) showssettler-decanters at pages 21-18 ff, mixers at 21-21ff, and dryers atpages 20-10ff.

Thereafter the mixture from the output 18 of the mixer 4 is introducedby a pump 7 into a pelletizer or nodulizer 6 to which water is fed at 12and pellets issuing from the output 19 thereof are fed to a rotary dryer11. Pelletizers are described on pages 8-61ff of the Chemical Engineers'Handbook and dryers on pages 20-17ff.

The arrangement of FIG. 2 differs from that of FIG. 1 in that the fluegas from the coal-fired power plant 8 is first passed through a cyclone10 (see pages 20-68ff op.cit.) to separate out dry fly ash which is fedthrough a line 20 to the mixer 4 where it is mixed with the particallydried sludge coming directly from the settler 3 and with water glass(Na₂ SiO₃ +H₂ O) fed in by a pump 5'. A travelling grate dryer 13 isused for the pellets (see pages 20-5 op.cit.). The use of dry fly ashfrom the cyclone 10 eliminates the need for the dryer 9. Some of theparticle-free flue gas is fed by a line 21 from the scrubber 1 to thedryer 13 to heat same.

In both plants as shown in FIGS. 1 and 2 the sludge at the output 15 ofthe scrubber has a water content, by weight, of approximately 70%. Thesettling tank reduces this by dewatering to approximately 30%, and thedryer in the apparatus of FIG. 1 lowers the content to about 15%.

Thus in accordance with the present invention the flue gas issuing fromthe plant 8 is mixed with pulverulent lime and scrubbed. The sludge soproduced is dryed and mixed with a binder, then the mixture ispelletized and hardened either by drying or burning in a neutralatmosphere.

The following Example illustrates the Invention.

EXAMPLE

A quantity of 141 kg of sludge drawn off the scrubber 1 is dried to amoisture content of 9.4%, and is treated with 21.1 kg Z 350 cement. Themixture then had a moisture content of 7.7%, and was pelletized in acommercial pelletizing drum 6. The unburnt pellets hardened only by airdrying had the following qualities:

    ______________________________________                                        water content (by weight)                                                                        14.8%                                                      bulk weight        1.5 kg/liter liter                                         strength after                                                                 1 day of air storage                                                                            11 kg/pellet 12 mm φ                                    3 days of air storage                                                                           16 kg/pellet 12 mm φ                                    56 days of air storage                                                                         166 kg/pellet 12 mm φ                                    Hardened in an autoclave                                                      after 1 day storage in air                                                                      70 kg/pellet 12 mm φ.                                  ______________________________________                                    

To determine the stability in water a soak test was conducted. Pelletsstored 9 days in air and 5 days in water yielded the following analysisfor the overlying water:

    ______________________________________                                        pH                12.6                                                        conductivity     3150 μ/cm                                                 SO.sub.4.sup.--   23 mg/liter                                                 SO.sub.3.sup.--   37 mg/liter                                                 Ca.sup.++         150 mg/liter                                                Total hardness    24.0° dH.                                            ______________________________________                                    

Subjecting the pellets to a burning process instead of drying in airgives them a considerable increase of the strength factor depending onthe burning procedure. In addition similar values are obtained by mixingthe sludge with flue dust, and reducing the content of hydraulic binder.

We claim:
 1. A plant for making hard and water-stable pellets usable asaggregate or filler, said plant comprising:means including a scrubbingtower for treating a waste gas containing pollutants with water and limeto form a sludge containing said pollutants; means connected to saidtower for partially dewatering said sludge; means connected to thedewatering means for mixing the dewatered sludge with a binder; meansconnected to the mixing means for pelletizing said sludge; and means forremoving particulate material from said gas prior to treatment thereofand for introducing said particulate material into the dewatered sludgeat said mixing means.
 2. The plant defined in claim 1, furthercomprising means for drying and hardening the pelletized sludge.
 3. Theplant defined in claim 2, further comprising means for feeding said gasafter treatment thereof with water and lime to the drying means.